In thermosyphon Borehole Heat Exchangers, a heat carrier fluid circulates while exchanging heat with the ground without the need of a circulation pump, representing an attractive alternative when compared to other more conventional systems. Normally, the fluid is at liquid-vapor saturation conditions and circulation is maintained by density differences between the two phases as the fluid absorbs energy from the ground. This paper presents some experimental experiences from a 65 meter deep thermosyphon borehole heat exchanger loop using Carbon Dioxide as heat carrier fluid, instrumented with a fiber optic cable for distributed temperature measurements along the borehole depth. The heat exchanger consists of an insulated copper tube through which the liquid CO2 flows downwards, and a copper tube acting as a riser. The results show temperatures every two meters along the riser, illustrating the heat transfer process in the loop during several heat pump cycles.

In this paper, the effects of applying magnetic field on hydrodynamics and heat transfer of Fe3O4/water magnetic nanofluid flowing inside a vertical tube have been studied experimentally. The applied magnetic field was resulted from quadrupole magnets located at different axial positions along the tube length. The variations of the local heat transfer coefficient and also the pressure drop of the ferrofluid flow along the length of the tube by applying the magnetic quadrupole field have been investigated for different Reynolds numbers. The obtained experimental results show maximum enhancements of 23.4%, 37.9% and 48.9% in the local heat transfer coefficient for the magnetic nanofluid with 2 vol% Fe3O4 in the presence of the quadrupole magnets located at three different axial installation positions for the Reynolds number of 580 and the relative increase in total pressure drop by applying the magnetic field is about 1% for Re = 580. The increase of the heat transfer coefficient is due to the radial magnetic force toward the heated wall generated by magnetic quadrupole field acting over the ferrofluid flowing inside the tube so that the velocity of the ferrofluid in the vicinity of the heated wall is increased. It is also observed that the enhancement of heat transfer coefficient by applying magnetic quadrupole is decreased with increasing the Reynolds number.

Experimental results on dryout of seven refrigerants (R134a, R1234yf, R152a, R22, R245fa, R290 and R600a) in small, single vertical tubes under upward flow conditions are reported in this study. The experiments were conducted under a wide range of operating conditions in stainless steel tubes (0.64-1.70. mm and 213-245. mm heated length). The effects of operating parameters like mass flux, vapor quality, saturation pressure and channel size are discussed in detail. In general, dryout heat flux increased with increasing mass flux, and with increasing tube diameter. No effect of varying saturation temperature was observed. The experimental findings were compared with well-known macro and micro-scale correlations from the literature and it was found that Wu's correlation (in modified form) quite satisfactorily predicted the whole database. A new correlation for prediction of heat flux at dryout conditions is also proposed.

This article reports on flow boiling heat transfer and dryout characteristics of R152a in a vertical mini-channel. The experiments were carried out with a resistively heated stainless steel tube (1.60mm in diameter and 245mm heated length) at 27 and 32°C saturation temperature. Five mass fluxes in the range 100-500kg/m2s with heat fluxes from 5 to 245kW/m2 were tested. Under similar operating conditions experiments were repeated with R134a in the same setup to compare thermal performance of R152a. The results showed that the heat transfer was strongly influenced by the applied heat flux with insignificant convective contributions. The dryout heat flux increased with increasing mass flux but no effect of varying operating pressure was noticed. The experimental results for heat transfer and dryout heat flux were compared with well-known macro and micro-scale correlations from the literature.

Flow boiling heat transfer, pressure drop and dryout characteristics of R1234yf in a vertical stainless steel test section (1.60mm inside diameter and 245mm heated length) under upward flow conditions are reported in this article. The experiments were carried out at 27 and 32°C saturation temperatures with five mass fluxes in the range of 100-500kg/m2s while the applied heat flux was in the range of 5-130kW/m2. The experiments were carried out with gradual increase of the applied heat flux til completion of dryout. Under similar conditions, tests were repeated with R134a in the same test setup to compare thermal performance of these two refrigerants. The results showed that boiling heat transfer was strongly controlled by the applied heat flux and operating pressure with insignificant dependence on mass flux and vapor quality. The frictional pressure drop increased with mass flux and vapor quality and decreased with increasing saturation temperature as expected. Signs of dryout first appeared at vapor qualities of 85%, with the values generally increasing with increasing mass flux. The effect of varying system pressure was insignificant. The experimental results (boiling heat transfer, pressure drop and dryout heat flux) were compared with the predictions from well-known correlations (for macro and micro-scale channels) from the literature.

Refrigerant-related environmental concerns forced legislative bodies to phase out some types of refrigerants, namely, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and in the near future European legislation will be affecting hydrofluorocarbons (HFCs) as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental findings on flow boiling heat transfer and dryout characteristics of isobutane (R600a) in a uniformly heated, vertical, stainless-steel test section (1.60mm inside diameter and 245mm heated length) are reported in this article. The experiments were conducted at two saturation pressures corresponding to the temperatures of 27 and 32 degrees C, with five mass fluxes in the range 50-350kg/m(2)-s and at outlet vapor qualities up to dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The dryout heat flux increased with increasing mass flux; however, no significant effect of varying saturation temperature was observed. The experimental results (for heat transfer and dryout) were compared with different macro and microscale correlations from the literature.

Refrigerant related environmental concerns forced legislative bodies to phase out some types of refrigerants namely CFC’s and HCFC’s and in the near future European legislation will be affecting HFCs as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental studies with these fluids are important in understanding their performance and potential. Experimental findings on flow boiling of Isobutane in a uniformly heated, vertical, stainless steel test section (1.6 mm inside diameter and 245mm heated length) are reported in this article. Experiments were conducted at two saturation pressures corresponding to the temperature of 27 and 32 oC, with five mass fluxes in the range 50-350 kg/m2s and at outlet vapour qualities up till dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The experimental results were compared with different macro and micro-scale correlations from the literature, and Owhaib, Liu & Winterton and Mikielewicz correlations quite accurately predicted the heat transfer data.

This study describes experimental findings on flow boiling heat transfer with R1234yf in a smooth, vertical stainless steel tube of 1.6 mm inner diameter and 245 mm heated length. Tests were conducted at two saturation pressures corresponding to saturation temperatures of 27 and 32 °C. Other operating parameters were: mass flux 100-500 kg/m²s with heat flux 3-65 kW/m² while quality change was up to 60%. The heat transfer coefficient appeared to be a strong function of the applied heat flux and insignificant effect of mass flux and quality was observed. Increase in saturation temperature/pressure increased the heat transfer performance. Experiments were repeated with R134a in the same test section to compare the two fluids, almost similar results were duplicated with R134a. Experimental results were compared with different correlations, Tran et al. (1996), Gungor and Winterton (1986) and Martín-Callizo et al. (2007) correlations accurately predicted the data.

Two phase heat transfer in small channels has many practical applications like, miniature heatexchangers, high powered electronics, miniature refrigeration system. Flow boiling in these compactchannels offers many potential advantages like, cope with high heat flux, less fluid inventory,compactness in size. It is well known that two phase heat transfer is drastically reduced when theheater surface becomes partially dry, for any reason. Moving beyond the point where this happensresults in a sharp increase in the temperature of the heated surface and eventually leads towardsburnout. So the upper operational limit (from safety and efficiency point of view) is extremelyimportant to be able to predict.Experimental findings on dryout of Isobutane in a uniformly heated, vertical, stainless steel testsection (1.6 mm inside diameter and 245mm heated length) are reported in this article. Experimentswere conducted at two saturation pressures corresponding to temperatures of 27 and 32 oC, with fivemass fluxes in the range 50-350 kg/m2s and with vapor fractions at the outlet up till dryout conditions.Analysis showed that the dryout heat flux increased with increasing mass flux, while no effect ofvarying the operating pressure was observed. Experimental results were compared with differentcorrelations from the literature, Wu [5], Mikielewicz [6], Callizo [3] and Katto-Ohno [4] correlationsquite satisfactorily predicted the data.

This study proposes a method and apparatus to estimate shelf stability of nanofluids. Nanofluids are fabricated by dispersion of solid nanoparticles in base fluids, and shelf stability is a key issue for many practical applications of these fluids. In this study, shelf stability is evaluated by measuring the weight of settled solid particles on a suspended tray in a colloid versus time and correlated with the performance change of some nanofluid systems. The effects of solid particle concentration and bath sonication time were investigated for selected nanofluids. The results show the applicability of this simple method and the apparatus to evaluate nanofluid shelf stability. Furthermore, it shows that Stokes' law is not valid for determining the settling time of the tested nanoparticles probably due to their complicated shape and presence of surface modifiers. The effect of shelf stability on thermal conductivity and viscosity was illustrated for some nanofluids. Experimental results show that water-based Al2O3 nanofluids have quite good shelf stability and can be good candidates for industrial applications.

This article reports convective single-phase heat transfer performance in laminar flow for some selected nanofluids (NFs) in an open small diameter test section. A 0.50 mm inner diameter, 30 cm long stainless steel test section was used for screening single phase laminar convective heat transfer with water and five different water based NFs. Tested NFs were; Al2O3 (two types), TiO2 (two types) and CeO2 (one type), all 9 wt.% particle concentration. The effective thermal conductivity of the NFs were measured with Transient Plane Source (TPS) method and viscosity were measured with a rotating coaxial cylindrical viscometer. The obtained experimental results for thermal conductivity were in good agreement with the predicted values from Maxwell equation. The local Shah correlation, which is conventionally used for predicting convective heat transfer in laminar flow in Newtonian fluids with constant heat flux boundary condition, was shown to be valid for NFs. Moreover, the Darcy correlation was used to predict the friction factor for the NFs as well as for water. Enhancement in heat transfer for NFs was observed, when compared at equal Reynolds number, as a result of higher velocity or mass flow rate of the NFs at any given Reynolds number due to higher viscosity for NFs. However, when compared at equal pumping power no or only minor enhancement was observed.

Nanofluids are engineered colloids of nanoparticlesdispersed homogenously in a base fluid, which theirthermophysical properties are changed by adding solidnanoparticles. Among the characteristic parameters,viscosity is one of the most important, as it directly affectsthe pumping power in cooling systems. In this study, theviscosity of water based Al2O3, ZrO2, and TiO2 (with 9wt%for all) nanofluids was measured and its impact on pressuredrop in a simple tubular pipe was estimated for bothlaminar and turbulent flow by classical correlations. Theeffect of temperature on the viscosity of these nanofluidswas also studied in the temperature range of 5˚C - 30˚C. Toassess the applicability of the classical correlations, pressuredrops across an open 30cm long, 0.50mm diameterstainless steel test section was measured for water andnanofluids by a differential pressure transducer. Theaverage viscosity increments compared to water in thetemperature range of 5˚C - 30˚C are 105%, 98% and 31% forAl2O3, ZrO2, and TiO2 nanofluids respectively. Moreover, theresults show that the viscosity of nanofluids decreases withthe increase of temperature; however the relative viscosity,which is defined as the viscosity ratio between a nanofluidand its base fluid is constant in 5˚C - 30˚C temperaturerange.

The advantages of using Al2O3, TiO2, SiO2 and CeO2 nanofluids as coolants have been investigated by analysing the combined effect of nanoparticles on thermophysical properties and heat transfer coefficient. The thermal conductivity and viscosity of these nanofluids were measured at two leading European universities to ensure the accuracy of the results. The thermal conductivity of nanofluids agreed with the prediction of the Maxwell model within +/- 10% even at elevated temperature of 50 oC indicating that the Brownian motion of nanoparticles does not affect thermal conductivity of nanofluids. The viscosity of nanofluids is well correlated by modified Krieger-Dougherty model providing that the effect of nanoparticles aggregation is taken into account. It was found that at the same Reynolds number the advantage of using a nanofluid increases with increasing nanofluid viscosity which is counterintuitive. At the same pumping power nanofluids do not offer any advantage in terms of cooling efficiency over base fluids since the increase in viscosity outweighs the enhancement of thermal conductivity.

Turbulent convective heat transfer coefficients of 9 wt% Al2O3/water and TiO2/water nanofluids inside a circular tube were investigated independently at the Royal Institute of Technology, KTH (Sweden) and at University of Birmingham (UK). The experimental data from both laboratories agreed very well and clearly show that Nusselt numbers are well correlated by the equations developed for single phase fluids with the thermophysical properties of nanofluid. The heat transfer coefficients of nanofluids can be compared with those of the base fluids at the same Reynolds number or at the same pumping power. As the same Reynolds number requires higher flow rate of nanofluids therefore such comparison shows up to 15% increase in heat transfer coefficient. However, at equal pumping power, the heat transfer coefficient of Al2O3 nanofluid was practically the same as that of water while that of TiO2 was about 10% lower. Comparing performance at equal Reynolds number is clearly misleading since the heat transfer coefficient can always be increased by increased pumping power, accordingly, the comparison between the fluids should be done at equal pumping power.

Attention has been given to enhance boiling surfaces in order to decrease the temperature difference and to increase heat transfer coefficient. Structured surfaces may provide both surface enlargement and artificial nucleation sites, thus ameliorate the heat transfer coefficient. The goal of the present experimental work is to analyze the influence on heat transfer coefficient (HTC) of enhanced surface structures coated on mini channel heat exchanger working in a closed loop thermosyphon system. Experimental tests were carried out with three types of surface enhanced mini channel evaporators: smooth surface, threaded structure and nanoporous coating. The evaporators are single channel half circularly shaped, adapted for filming purpose, measuring 30mm in length and 3mm in diameter. Surface areas of channels are 1.41cm(2). Experiments were conducted in refrigerant 134a at 4.87bar (reduced pressure pr=0.12) and at heat fluxes ranging from 0.7W/cm(2) to 63.8W/cm(2). A high speed video camera was used for visualization of the two-phase flow in the evaporator channel. It is shown that threaded surface provides the highest heat transfer coefficient (HTC) from no load to heat flux of 7.1W/cm(2), the nanoporous structure shows the highest performance between 7.1W/cm(2) and 49.6W/cm(2), and the smooth surface channel exhibits the best HTC from 49.6W/cm(2) and higher. In this paper, the influences of heat flux and surface structures on HTC are discussed, and the impact of refrigerant flow regimes on heat transfer performance is also highlighted.

Computerized simulation technique is a fundamental component of the higher education and is a vital mechanism, utilized in phenomena-graphic studies in science. The system offers recreation of an alternative reality in front of scientist's eyes, giving an insight into the biggest and smallest scales (astronomy to nanotechnology) yet cannot be comprehended otherwise. Currently the tool is utilized in research and taught one on one. The progress in computational technology and the advent of commercial codes equipped with user-friendly interface have facilities the introduction of computational fluid dynamic in undergraduate education promoting critique of various learning opportunities through visualization technique. Nevertheless the outcomes of each study is highly influenced by the ideas, approaches, knowledge in phenomena, mathematical rules and measures, computer science, cads, post processing and interpretation of the results. The user of such system acts as the facilitator and liaison between the real and virtual phenomena. The user should own the proper education, experience, emphasizing the relevance of the teaching strategy; understanding the key mechanisms in learning process. Unlocking the full power of computational fluid dynamic, some critical topics in educational area need to be addressed. One is to identify the learning process, approaches and methodology. One task with three different definitions was given to groups and the data regarding the learning process, strategies for solving the problem adapted by students for, engagements of the participants in work was monitored. The data were gathered through a net based learning tool. The progress of the groups were scrutinized weekly, aiming at directing the group in pre-define learning scope. This work is based on instructional design and andragogically approach. It offers an elaboration on the mechanism of learning process and is premeditated in context of prescribed framework. The results indicated the prominence of student sensitive and constructive learning process and the advantages of using a preferred framework in guiding the students in pertinent context (area) particularly incorporation of constructivist principles that may lead to enhance learner's learning experience. In addition, the conclusions of this study similarly illuminate the vast potentials of computational fluid dynamic for research, evaluation and educational purposes.

Following is an experimental study of six different evaporators in a closed two-phase thermosyphon loop system, where the influence of various evaporator dimensions and surfaces was investigated. The evaporators featured a 30 mm long rectangular channel with hydraulic diameters ranging from 1.2-2.7 mm. The heat transfer surface of one of the tested evaporators was enhanced with copper nano-particles, dendritically connected into an ordered micro-porous three dimensional network structure. To facilitate high speed video visualization of the two-phase flow in the evaporator channel, a transparent polycarbonate window was attached to the front of the evaporators. Refrigerant 134A was used as a working fluid and the tests were conducted at 6.5 bar. The tests showed that increasing channel diameters generally performed better. The three largest evaporator channels exhibited comparable performance, with a maximum heat transfer coefficient of about 2.2 W/(cm(2)K) at a heat flux of 30-35 W/cm(2) and a critical heat flux of around 50 W/cm(2). Isolated bubbles characterized the flow regime at peak performance for the large diameter channels, while confined bubbles and chaotic churn flow typified the evaporators with small diameters. In line with previous pool boiling experiments, the nucleate boiling mechanism was significantly enhanced, tip to 4 times, by the nano- and micro-porous enhancement structure.

An experimental investigation and theoretical study of thermal conductivity and viscosity of Al2O3/water nanofluids are presented in this article. Various suspensions containing Al2O3 nanoparticles were tested in concentration ranging from 3% to 50% in mass and temperature ranging from 293K to 323K. The results reveal that both the thermal conductivity and viscosity of nanofluids increase with temperature and particle concentration accordingly while the increase in viscosity is much higher than the increase in thermal conductivity. The thermal conductivity and viscosity enhancement are in the range of 1.1-87% and 18.1-300%, respectively. Moreover, the results indicate that the thermal conductivity increases nonlinearly with concentration, but, linearly with the increase in temperature. In addition, the experimental results are compared with some existing correlations from literature and some modifications are suggested. Finally, the average heat transfer coefficient at different basis of comparisons including equal Reynolds number, fluid velocity and pumping power is studied based on the experimental thermal conductivity and viscosity in fully developed laminar and turbulent flow regimes. It is found that equal Reynolds number as a basis of comparison is highly misleading and equal pumping power can be used to study the advantage of using nanofluid instead of the base fluid.

Thermal performance of cylindrical heat pipe with nanofluid is studied based on the laws of thermodynamics. The objective of the present work is to investigate nanofluids effect on different sources of entropy generation in heat pipe caused by heat transfer between hot and cold reservoirs and also frictional losses and pressure drop in the liquid and vapor flow along heat pipe. An analytical study was performed to formulate all sources of entropy generation and the predicted results are compared with experimental ones. Cylindrical miniature grooved heat pipes of 250 mm length and 6.35 mm outer diameter were fabricated and tested with distilled water and water based TiO2 and Al2O3 nanofluids at different concentrations as working fluids. Analytical and experimental results revealed that the entropy generation in heat pipes decreases when nanofluids are used as working fluids instead of basefluid which results in improved thermal performance of the heat pipes with nanofluids.

In this study, effect of Al2O3 nanofluid on thermal performance of cylindrical heat pipe is investigated. An analytical model is employed to study the thermal performance of the heat pipe utilizing nanofluid and the predicted results are compared with the experimental results. A substantial change in the heat pipe thermal resistance, effective thermal conductivity and entropy generation of the heat pipe is observed when using Al2O3 nanofluid as a working fluid. It is found that entropy generation in the heat pipe system decreases when using a nanofluid due to the lower thermal resistance of the heat pipe which results in an improved thermal performance. It is shown that the proposed model is in reasonably good agreement with the experimental results and can be used as a fast technique to explore various features of thermal characteristics of the nanofluid based heat pipe.

An experimental study was performed to investigate the thermal performance of heat pipes using SiC/water nanofluid as the working fluid. Four cylindrical copper heat pipes containing two layers of screen mesh were fabricated and tested with water and water based SiC nanofluids with nanoparticle mass concentrations of 0.35%, 0.7% and 1.0% as working fluids. SiC nanofluids properties and characteristics are evaluated and its effects on thermal performance improvement of screen mesh heat pipes at different concentrations and inclination angles are investigated. Experimental results show that nanofluid improves the performance of the heat pipes and the thermal resistance of the heat pipe with SiC nanofluid decreases with increasing nanoparticle concentration. Thermal resistance reduction of heat pipes by 11%, 21% and 30% was observed with SiC nanofluids containing 0.35 wt.%, 0.7 wt.% and 1.0 wt.% SiC nanoparticles as compared with water. In addition, it is revealed that the inclination angle has remarkable influence on the thermal performance of the heat pipes and the lowest thermal resistance belongs to the inclination angle of 60 in all concentrations. The present investigation indicates that the maximum heat removal capacity of the heat pipe increases by 29% with SiC nanofluids at nanoparticle mass concentration of 1.0 wt.%.

In this study theoretical evaluation of performance of a three ice based cool thermal energy storage systems is conducted ; (a) static, indirect, external melt ice-on-coil ; (b) dynamic ice slurry type storage with a water and (c) ice slurry distribution system. In order to investigate and assess possible economic and energy saving potential of an ice slurry storage system over conventional static type a computer simulation models were used. The systems were compared for high temperature application, for the purpose of milk cooling in the dairy industry. The product temperature that has to be achieved is +3 °C which requires a secondary coolant temperature to be less than +1 °C. Calculations have been performed on basis of specific user supplied load data for a design day, acquired as an actual case for dairy plant Prehrambeno industrijski kombinat (PIK) in the city of Rijeka, Croatia, and local electricity billing rate structure. The comparison shows that the dynamic cool thermal energy storage system (CTES) is favourable as to energy consumption in all studied cases.

Pressure drop behaviour of ice slurry based on ethanol-water mixture in circular horizontal tubes has been experimentally investigated. The secondary fluid was prepared by mixing ethyl alcohol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 degrees C). The pressure drop tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 30% depending on test conditions. Results from flow tests reveal much higher pressure drop for higher ice concentrations and higher velocities in comparison to the single phase flow. However for ice concentrations of 15% and higher, certain velocity exists at which ice slurry pressure drop is same or even lower than for single phase flow. It seems that higher ice concentration delay flow pattern transition moment (from laminar to turbulent) toward higher velocities. In addition experimental results for pressure drop were compared to the analytical results, based on Poiseulle and Buckingham-Reiner models for laminar flow, Blasius. Darby and Melson, Dodge and Metzner, Steffe and Tomita for turbulent region and general correlation of Kitanovski which is valid for both flow regimes. For laminar flow and low buoyancy numbers Buckingham-Reiner method gives good agreement with experimental results while for turbulent flow best fit is provided with Dodge-Metzner and Tomita methods. Furthermore, for transport purposes it has been shown that ice mass fraction of 20% offers best ratio of ice slurry transport capability and required pumping power.

Heat transfer of ice slurry flow based on ethanol-water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 degrees C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus-Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. it was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus-Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re-cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results. (C) 2009 Elsevier Ltd and IIR. All rights reserved.

Nano scale solid particles dispersed in base fluids are a new class of engineered colloidal solutions called nanofluids. Several studies reported enhancement of heat transfer by using nanofluids. This article reports convective single-phase heat transfer coefficients in an open 30 cm long, 0.50 mm internal diameter stainless steel test section. The setup is used for screening single phase laminar convective heat transfer with water and three different nanofluids: water based Al2O3, ZrO2, and TiO2 (all with 9 wt% of particles). A syringe pump with adjustable pumping speed is used to inject fluids into the test section. Thirteen T-type thermocouples are attached on the outer surface of the test section to record the local wall temperatures. Furthermore, two T-type thermocouples are used to measure inlet and outlet fluid temperatures. A DC power supply is used to heat up the test section and a differential pressure transducer is used to measure the pressure drop across the tube. Furthermore, the effective thermal conductivities of these nanofluids are measured using the Transient Plane Source (TPS) method at a temperature range of 20 - 50 degrees C. The experimental average values of heat transfer coefficients for nanofluids are compared with water. Enhancement in heat transfer of nanofluids is observed only when compared at constant Reynolds number (Due to higher viscosity for nanofluids, higher velocity or mass flow rate is required for nanofluids to reach the same Reynolds number). The other methods of comparison: equal mass flow rate, volume flow rate, pressure drop and pumping power did not show any augmentation of the heat transfer coefficient for the tested nanofluids compared to water.

Thermal conductivity and viscosity of alumina (Al2O3), zirconia (ZrO2), and titania (TiO2) nanofluids (NFs) were measured at 20°C. All the NF systems were water based and contained 9wt.% solid particles. Additionally, the heat transfer coefficients for these NFs were measured in a straight tube of 1.5m length and 3.7mm inner diameter. Based on the results, it can be stated that classical correlations, such as Shah and Gnielinski, for laminar and turbulent flow respectively, can be employed to predict convective heat transfer coefficients in NFs, if the accurate thermophysical properties are used in the calculations. Convective heat transfer coefficients for NFs were also compared with those of the base fluids using two different bases for the comparison, with contradictory results: while compared at equal Reynolds number, the heat transfer coefficients increased by 8-51%, whereas compared at equal pumping power the heat transfer coefficients decreased by 17-63%. As NFs have higher viscosity than the base fluids, equal Reynolds number requires higher volumetric flow, hence higher pumping power for the NFs. It is therefore strongly suggested that heat transfer results should be compared at equal pumping power and not at equal Reynolds number.

This article investigates the influence of temperature, concentration, and size of nanoparticles, and addition of surfactants on dynamic viscosity of water-based nanofluids containing alumina (Al2O3) and titania (TiO2) nanoparticles. Two viscometers, a capillary and a falling ball, were used for the measurements in the temperature range of 20-50 A degrees C and the particle concentration of 3-14.3 wt.%. The results indicate that the viscosity of nanofluids is reduced by increasing the temperature, similar to their base fluids. Moreover, surfactants, which are used to improve the shelf stability of nanofluids, most likely increase their viscosity. The correlations derived from the linear fluid theory such as Einstein and Batchelor, especially for solid concentration above 1.5 wt.% are not accurate to predict viscosity of nanofluids, while the modified Krieger-Dougherty equation estimates viscosity of nanofluids with acceptable accuracy in a specific range of solid particle size to aggregate size.

As heat generation from electronic components increase and the limit of air-cooling is reached, the interest for using liquid cooling for high heat flux applications has risen. Thermosyphon cooling is an alternative liquid cooling technique, in which heat is transferred as heat of vaporization from evaporator to condenser with a relatively small temperature difference. The effect of fluid properties, the structure of wall surfaces, and the effect of system pressure was investigated and reported previously by the author. In this paper, the influence of heat flux, system pressure, mass flow rate, vapor fraction, diameter of evaporator channel and tubing distance between evaporator and condenser on the heat transfer coefficient of an advanced two-phase thermosyphon loop is reported. The tested evaporators were made from small blocks of copper with 7, 5, 4, 3 and 2 vertical channels with the diameters of 1.1, 1.5, 1.9, 2.5, and 3.5 mm, respectively and the length of 14.6 mm. Tests were done with isobutane at heat fluxes ranging between 28.3 and 311.5 kW/m(2).

This study presents an experimental investigation of pressure drop in the evaporators and the riser of an advanced thermosyphon loop. The thermosyphon was designed for the cooling of three parallel high heat flux electronic components. The tested evaporators were made from small blocks of copper in which 7, 5, 4, 3, 2, 1 vertical channels with the diameters of 1.1, 1.5, 1.91 2.5, 3.5 and 6 mm, respectively, and a length of 14.6 mm were drilled. Tests were done with isobutane at heat fluxes ranging between 22.4 and 303 kW/m(2). For prediction of the pressure drop, in the riser, different combinations of frictional pressure drop and void fraction correlations were tested. Regarding the evaporator a simple correlation based on a homogeneous model [M.B. Bowers, I. Mudawar, Two-phase electronic cooling using mini-channel and macro-channel heat-sinks-part 11, flow rate and pressure drop constraints, ASME J Electron Packaging H 6 (1994) 298-305. [1]] has been used to predict the pressure drop.

ln this investigation an advanced thermosyphon loop with extended evaporator and condenser surfaces has been tested at high heat fluxes. The thermosyphon investigated is designed for the cooling of three parallel high heat flux electronic components. The tested evaporators were made from small blocks of copper in which five vertical channels with a diameter of 1.5 mm and length of 14.6 mm were drilled. The riser and downcomer connected the evaporators to the condenser, which is an air-cooled roll-bond type with a total surface area of 1.5 m(2) on the airside. Tests were done with Isobutane (R600a) at heat loads in the range of 10-90 W/cm(2) to each of the components with forced convection condenser cooling and with natural convection with heat loads of 10-70 W.

Two-phase flow instabilities which may occur at low and high heat loads were studied for a thermosyphon loop with R134a as refrigerant. The heat transfer surface of the evaporator was enhanced with a copper nano- and micro-porous structure. The heat transfer of the enhanced evaporator was compared to a smooth surface evaporator. Finally, the influence of the liquid level and the inside diameter of the riser on the instability of the system have been investigated. It was found that the enhanced structure surface decreased the oscillations at the entire range of heat fluxes and enhanced the heat transfer coefficient. Three flow regimes were observed: Bubbly flow with nucleate boiling heat transfer mechanism, confined bubbly/churn flow with backflow and finally churn flow at high heat fluxes.

In this study, the influence of different channel geometries on heat transfer, flow regime and instability of a two-phase thermosyphon loop, is investigated. Instabilities in flow regime and heat transfer, at low and high heat fluxes, are observed. Bubbly flow with nucleate boiling heat transfer mechanism, confined bubbly/slug flow with backflow for small channel height (H) and finally slug/churn flow at high heat fluxes are observed. This study shows that flow and thermal instability increases as channel height (H) decreases and also heat transfer coefficient increases with increasing channel height and heat flux. Bubbly flow characterizes the flow regime at high heat transfer coefficients while confined bubbles, backflow and intermittent boiling are more significant for low channel heights with lower heat transfer coefficient and critical heat flux. (C) 2010 Elsevier Ltd. All rights reserved.

KTH, School of Industrial Engineering and Management (ITM), Energy Technology.

COOLING OF CPU WITH A THERMOSYPHON2008In: 2008 SECOND INTERNATIONAL CONFERENCE ON THERMAL ISSUES IN EMERGING TECHNOLOGIES - THEORY AND APPLICATION (THETA), NEW YORK: IEEE , 2008, p. 239-243Conference paper (Refereed)

Abstract [en]

In this study the tested evaporator is made from small blocks of copper with I I vertical channels with a diameter of 2.5 mm and length of 30 mm. The riser and downcomer connected the evaporator to the condenser, which is cooled by air in free or forced convection. The condenser is made from 10 mm aluminium heat sink profile of the size of 365x365 mm with a fin length of 20 mm at a distance of 8 mm. In the top part of the heat sink a condenser channel system with 2x2 mm cross section is milled. The CPU used in this study is an Intel Pentium 4 with 3.2 GHz. The maximum heat load to the processor is 104W, and the highest temperature allowed on the processor is 65 degrees C Temperatures are measured for idle, 50% and maximum heat load of the processor at the CPU, evaporator wall, condenser wall and in the ambient. Temperature differences in the thermosyphon system, between the CPU and the evaporator wall, the condenser wall and ambient are presented. Isobutane has been used as the working fluid, due to the fact that it has low saturation pressure and that this refrigerant is friendly to the environment.

One efficient method of cooling electronics is to use a closed-loop two-phase thermosyphon system. The setup tested here utilizes three small evaporators connected in parallel, each made from a small block of copper in which five vertical channels with diameter 1.5 mm and length 15 mm were drilled. The article presents the experimental results in terms of heat transfer coefficients of smooth surfaces as well as for threaded surfaces. Tests were done at different heat fluxes while maintaining constant system pressure. Tests were performed with heat loads of 30-450 W dissipated through the system. Two different refrigerants, R134a and R600a, were tested. The experimental two-phase flow heat transfer coefficients were compared to correlations from the literature.

In a global effort to mitigate global warming refrigerants with a high Global Warming Potential (GWP) will be banned in certain applications. The current European legislation prohibits a refrigerant R404A, which is commonly used in retail food refrigeration systems, from placing on the European market from the 1st of January 2020. Similarly, R404A will be soon unacceptable in retrofit and new supermarket systems in the USA. A refrigerant R449A is a potential replacement for R404A but with the GWP that is significantly lower than R404A's. In this study experimental investigations are conducted during the conversion of a supermarket refrigeration system from R404A to R449A. The performance data is presented and environmental benefits of the conversion are discussed.

Recent legislative measures promote use of environmentally friendly refrigerants to mitigate global warming. Different environmental metrics are used to facilitate decision-making process of selection of refrigerant with low environmental impact. Total equivalent warming impact (TEWI) methodology is able to reflect the effect of heat pump or refrigeration system to global warming by accounting for both direct and indirect emissions associated with the refrigeration system. This paper extends TEWI methodology by using Global Temperature change Potential (GTP) metric instead of Global Warming Potential (GWP) one, which conventionally used to put different emissions on single CO2 scale. The analysis is done for the simple vapour compression heat pump system, where a number of refrigerants are compared. The study concludes with the analysis of applicability of modified TEWI analysis for further refrigerating system environmental assessment studies.

Recent legislative measures promote use of environmentally friendly refrigerants to mitigate global warming. Different environmental metrics are used to facilitate decision-making process of selection of refrigerant with low environmental impact. Total equivalent warming impact (TEWI) methodology is able to reflect the effect of heat pump or refrigeration system to global warming by accounting for both direct and indirect emissions associated with the refrigeration system. This paper extends TEWI methodology by using Global Temperature change Potential (GTP) metric instead of Global Warming Potential (GWP) one, which conventionally used to put different emissions on single CO2 scale. The analysis is done for the simple vapour compression heat pump system, where a number of refrigerants are compared. The study concludes with the analysis of applicability of modified TEWI analysis for further refrigerating system environmental assessment studies.

According to the requirements of the European regulation on fluorinated greenhouse gases, refrigeration equipment that contain high Global Warming Potential (GWP) refrigerants that have GWP equal or greater than 2500, with some exceptions, will be prohibited from placing on the European market from 1st January 2020. Moreover, the use of such fluorinated greenhouse gases to service or maintain refrigeration equipment with a charge size of 40 tonnes of CO2 equivalent or more will be prohibited as well. The greatest impact of the Regulation in the nearest time will be therefore on users of R404A refrigerant, which is the most popular refrigerant for supermarket refrigeration systems. Equipment users and manufacturers are therefore looking for the alternative refrigerants to replace R404A in new and existing equipment. R448A and R449A are potential R404A replacements. Both are non-toxic and non-flammable refrigerant blends that closely match the properties of R404A and have GWPs significantly lower than R404A. Cycle performance of a supermarket refrigeration system, operated with R448A, R449A and R404A is modelled in this study.

According to the requirements of the European regulation on fluorinated greenhouse gases, refrigeration equipment that contain high Global Warming Potential (GWP) refrigerants that have GWP equal or greater than 2500, with some exceptions, will be prohibited from placing on the European market from 1st January 2020. Moreover, the use of such fluorinated greenhouse gases to service or maintain refrigeration equipment with a charge size of 40 tonnes of CO2 equivalent or more will be prohibited as well.

The greatest impact of the Regulation in the nearest time will be therefore on users of R404A refrigerant, which is the most popular refrigerant for supermarket refrigeration systems. Equipment users and manufacturers are therefore looking for the alternative refrigerants to replace R404A in new and existing equipment.

R448A and R449A are potential R404A replacements. Both are non-toxic and non-flammable refrigerant blends that closely match the properties of R404A and have GWPs significantly lower than R404A. Cycle performance of a supermarket refrigeration system, operated with R448A, R449A and R404A is modelled in this study.